The nuclear fuel cycle refers to the progression of nuclear fuel from its beginning stages of extraction of uranium from the earth, converting that uranium to usable energy, and then on to the end stages of spent fuel storage and reprocessing. Spent fuel rods can either be stored in a repository or reprocessed to extract uranium and recycle it back into usable fuel. The reprocessing technique most commonly used in the nuclear fuel cycle is called the Plutonium Uranium Redox Extraction (PUREX) process, and one step consists of dissolving spent fuel rods in hot nitric acid for ion recovery. Uranyl peroxide nanoclusters are cage-like clusters of uranyl ions bound together by peroxide and hydroxyl bridges. The inherent bent angle of the peroxide bond is what gives these clusters their spherical shape. There are over 120 unique cage clusters that contain 16 to 124 uranyl polyhedra with diameters ranging from 1.5 to 4.0 nm. They readily form in solutions that contain a uranyl species, peroxide, a cation source, and an alkaline pH. The goal of our research project is to investigate the dissolution of insoluble uranium-bearing minerals by introducing them to nanocluster forming conditions. We are investigating uranyl silicate minerals in the uranophane group because they are insoluble, only releasing 10 to 100 ppm of uranium into solution when in contact with water. The uranophane group contains minerals that are sheet structures made up of made up of uranyl and silicate polyhedra with cations and water in the interlayer. By changing the interstitial cations in the mineral, we are able to investigate the effects these cations could have on the dissolution process. By introducing pure phases of the mineral to peroxide at concentrations of 0.01M to 1M and pH conditions ranging between 7 and 11, we can observe whether pH, peroxide concentration, or combinations of each have the greatest effect on the uranium dissolution. After a week of continuous mixing and daily pH measurements, each solution is analyzed for the presence of nanoclusters and the amount of uranium released from the mineral. Thus far, when uranophane, Ca(UO2)2(SiO3OH)2 ● 5H2O, has been introduced to these conditions—mild compared to those in the PUREX reprocessing system— it has released over 6,000 ppm of uranium into solution in the form of nanoclusters, compared to 100 ppm in just water alone.

Dissolution of uranyl silicate minerals in uranyl peroxide nanocluster forming environments

253rd National Meeting (2017)

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